A reciprocating engine to be employed in a motor vehicle, a motorcycle, an agricultural machine, a marine vessel or the like requires a crankshaft to extract power by converting reciprocating motions of pistons to rotational motion. There are two types of crankshafts: the type manufactured by die forging and the type manufactured by casting. Especially when high strength and high stiffness are required, die forged crankshafts (which will hereinafter be referred to as “forged crankshafts”) are often employed.
FIGS. 1A to 1C are schematic diagrams showing an example of a shape of a commonly used crankshaft. FIG. 1A is an overall view, FIG. 1B is a sectional view along the line IB-IB, and FIG. 1C shows the phases of pins. In order to facilitate understanding of the shape of the crankshaft, FIG. 1B shows only a crank arm A1, a counterweight W1 integrated with the crank arm A1, a pin P1 and a journal J1 connected to the crank arm A1, which are extracted from the crankshaft.
The crankshaft 11 shown in FIGS. 1A to 1C is a four-counterweight crankshaft to be mounted in a three-cylinder engine. The crankshaft 11 includes four journals J1 to J4, three pins P1 to P3, a front part Fr, a flange Fl, and six crank arms (hereinafter referred to simply as “arms”) A1 to A6. The six arms A1 to A6 connect the journals J1 to J4 respectively to the pins P1 to P3. Some of the six arms A1 to A6 have counterweights (hereinafter referred to simply as “weights”) W1 to W4, respectively, which are integrated therewith. Specifically, the first arm A1, the second arm A2, the fifth arm A5 and the sixth arm A6 incorporate the weight W1, W2, W3 and W4, respectively. The third arm A3 and the fourth arm A4 do not have weights.
The front part Fr is located at a front edge of the crankshaft 11, and the flange Fl is located at a rear edge of the crank shaft 11, the front edge and the rear edge being edges in a direction along the axis of the crankshaft 11. The front part Fr is connected to the front first journal J1, and the flange Fl is connected to the rearmost fourth journal J4.
In the following paragraphs, when the journals J1 to J4, the pins P1 to P3, the arms A1 to A6, and the weights W1 to W4 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the pins, a reference character “A” for the arms, and a reference character “W” for the weights. An arm A and a weight W integrated therewith are referred to collectively as a “web”.
As shown in FIG. 1C, the pins P1 to P3 are arranged at intervals of 120 degrees around the journals. In other words, each of the pins P1 to P3 is located at a first position L1, a second position L2 or a third position L3, and the phase differences among the first to the third positions L1 to L3 are 120 degrees.
As shown in FIG. 1B, the width Bw of the weights W is greater than the width Ba of the arms A. Accordingly, each of the weights W bulges greatly from an arm center plane (a plane including the axis of the pin P and the axis of the journal J).
A forged crankshaft having such a shape is generally produced by using a billet as a starting material. A section of the billet in a direction perpendicular to the longitudinal direction thereof, that is, a cross section of the billet is circular or square, and the cross-sectional area is constant throughout the length. In the following paragraphs, a section of a crankshaft in a direction perpendicular to the axis of the crankshaft is referred to as a “cross section”, and a section of the crankshaft in a direction parallel to the axis of the crankshaft is referred to as a “longitudinal section”. The area of the cross section is referred to simply as a “sectional area”. A method for producing a forged crankshaft includes a preforming step, a die forging step, and a trimming step that are to be executed in this order. After the trimming step, a coining step may be executed if needed. Typically, the preforming step includes a rolling step and a bending step, and the die forging step includes a rough forging step and a finish forging step.
FIGS. 2A to 2F are schematic diagrams showing a conventional method for producing a common forged crankshaft. FIG. 2A shows a billet, FIG. 2B shows a rolled blank, FIG. 2C shows a bent blank, FIG. 2D shows a rough forged blank, FIG. 2E shows a finish forged blank, and FIG. 2F shows a forged crankshaft. FIGS. 2A to 2F show a method for producing a crankshaft having the configuration shown in FIGS. 1A to 1C.
In the production method shown in FIGS. 2A to 2F, a forged crankshaft 11 is produced as follows. First, a billet 12 with a specified length as shown in FIG. 2A is heated in a heating furnace, and in a preforming step, the heated billet is rolled and subsequently bent. In the rolling, the billet 12 is rolled and reduced, for example, by grooved rolls. This is to distribute the volume of the billet 12 in the axial direction, and thereby, a rolled blank 13, which is an in-process material, is obtained (see FIG. 2B). Next, in the bending, the rolled blank 13 is partly pressed and reduced from a direction perpendicular to the axial direction. This is to distribute the volume of the rolled blank 13, and thereby, a bent blank 14, which is a next in-process material, is obtained (see FIG. 2C).
Next, in a rough forging step, the bent blank 14 is forged by a pair of an upper die and a lower die, and thereby, a rough forged blank 15 is obtained (see FIG. 2D). The rough forged blank 15 is roughly in the shape of the crankshaft (final product). In the finish forging step, the rough forged blank 15 is forged by a pair of an upper die and a lower die, and thereby, a finish forged blank 16 is obtained (see FIG. 2E). The finish forged blank 16 has a shape in agreement with the shape of the finished crankshaft. During the rough forging and the finish forging, excess material flows out through a space between the mutually facing parting faces of the dies, which results in formation of flash B. Accordingly, the rough forged blank 15 and the finish forged blank 16 have great flash B on the periphery.
In a trimming step, for example, while the finish forged blank 16 is nipped and held by a pair of dies, the finish forged blank 16 is punched by a cutting die. Thereby, the flash B is removed from the finish forged blank 16, and a forged blank with no flash is obtained. The forged blank with no flash has substantially the same shape as the forged crankshaft 11 shown in FIG. 2F.
In a coining step, main parts of the forged blank with no flash are slightly pressed by dies from above and below so that the forged blank with no flash can have the exact size and shape of the final product. The main parts of the forged blank with no flash are, for example, shaft parts such as the journals J, the pins P, the front part Fr, the flange Fl and the like, and further, the arms A and the weights W. In this way, the forged crankshaft 11 is produced. It is noted that, when the crankshaft to be produced is a three-cylinder four-counterweight crankshaft or the like wherein the pins are arranged around the journals at intervals of 120 degrees, after the trimming step, a twisting step may be additionally executed for adjustment of the placement angles of the pins.
The production method shown in FIGS. 2A to 2F are applicable not only to production of a three-cylinder four-counterweight crankshaft as shown in FIGS. 1A to 1C but also to production of any other crankshaft. For example, crankshafts to be mounted in four-cylinder engines, in-line six-cylinder engines, V-type six-cylinder engines, eight-cylinder engines and others can be produced by the same production method.
The main purpose of the preforming step is distributing the volume of the billet, and therefore, the blank obtained thereby is hardly in the form of the forged crankshaft. By distributing the volume of the billet in the preforming step, it is possible to decrease the outflow of material and accordingly to decrease the formation of flash in the next die forging step, thereby improving the material yield rate. The material yield rate means the rate (percentage) of the volume of the forged crankshaft (final product) to the volume of the billet.
For example, Japanese Patent Application Publication No. 2001-105087 (Patent Literature 1), Japanese Patent Application Publication No. H2-255240 (Patent Literature 2) and Japanese Patent Application Publication No. S62-244545 (Patent Literature 3) disclose techniques relating to production of a forged crankshaft. Patent Literature 1 teaches a preforming step using a pair of an upper die and a lower die. During pressing of a rod-like workpiece by use of an upper die and a lower die in the preforming step, while a part of the workpiece is elongated, another part connecting thereto is offset from the axis. In the preforming step disclosed in Patent Literature 1, rolling and bending are performed at the same time, which allows a decrease in investment for facilities.
According to Patent Literature 2, in a preforming step, a four-pass high-speed rolling, rather than a conventional two-pass rolling, is performed. A rolled blank obtained by the preforming step has sectional areas that are congruent with the sectional area distribution among weights, arms and journals of the forged crankshaft (final product). According to Patent Literature 2, this improves the material yield rate.
According to Patent Literature 3, in a preforming step, a billet is pressed while being nipped by at least two dies that are movable relative to each other. By rolling operation of the dies, the material of the billet is distributed in the axial direction and the radial direction. Thereby, a blank having a shape that is asymmetric about the axis and is congruent with the general shape of the crankshaft to be produced can be obtained. In the production method disclosed in Patent Literature 3, a blank having a shape that is asymmetric about the axis can be obtained only by the preforming step, which allows direct advancement to a die forging step.